The principle and five major components of a laser barcode scanning gun
Laser barcode scanners have gained widespread use due to their unique advantages, including a large depth of field, high scanning speed, and a wide scanning range. In addition, laser full-angle barcode scanners are extensively used in highly automated, high-volume logistics fields because they can quickly scan and read barcode symbols in any direction. Laser barcode scanners consist of several components, including the laser source, optical scanning, optical reception, photoelectric conversion, signal amplification, shaping, quantization, and decoding. Below, we will discuss these components in detail.
Principle of Laser Scanners
Laser scanners work by scanning barcodes using emitted light. They identify barcodes by the significant differences in light reflected by the black and white bars and spaces of the barcode. When scanning a set of barcodes, the light source is projected onto the barcode, and the reflected light passes through a lens, focusing it onto the scanning module (commonly referred to as a scanner decoder). The scanning module then converts the light signal into an analog-to-digital signal (voltage), which corresponds to the received light's intensity. This information is then transmitted to the computer as the desired barcode content. During this process of capturing the light source, decoding, and conversion into computer input signals, if the barcode cannot be correctly recognized, the laser source remains on, indicating that the scanner is continually trying to decode. If decoding is successful, the laser light turns off automatically.
At this point, the analog-to-digital conversion circuit converts the analog voltage into digital signals and sends them to the computer. Color is quantized using RGB three-color codes of 8, 10, or 12 bits, turning the signal into the aforementioned bit-depth image output. Higher quantization bits mean that the image can have richer detail and depth. However, it's essential to note that color ranges beyond the capability of human eyes. Within the distinguishable range, a higher bit-depth scanner provides smoother color transitions and reveals more fine details in the image.
Components of a Laser Scanner
(1) Laser Source
Visible light semiconductor lasers produced using Metal Organic Vapor Phase Epitaxy (MOVPE) technology offer various advantages, such as low power consumption, direct modulation, compact size, light weight, high reliability, and high efficiency. They quickly replaced the previously used He-Ne lasers as soon as they were introduced.
The laser beam emitted by semiconductor lasers is elliptical and non-axisymmetric. The divergence angle of the emitted beam perpendicular to the P-W plane is approximately 30°, and parallel to the plane is about 10°. Traditional beam shaping technology may cause the long and short axes of the elliptical beam spot to swap when the beam converges on two sides. This reduces the scanner's depth of field significantly. To overcome this issue, a beam shaping technology similar to that shown in Figure 3 is used to create a compact elliptical beam with an excellent depth of field. This elliptical beam is suitable for single-line laser barcode scanners, as it's less sensitive to printing noise and delivers better performance than circular beam characteristics.
For full-angle barcode laser scanners, as the beam may sometimes scan the barcode at a considerable tilt, a circular beam shape is preferable. Typically, this circular beam shape is created by adding a small circular aperture stop in front of the collimating lens. This beam characteristic can be well approximated by the diffraction properties of the small aperture. Using this approach, the depth of field for standard UPC barcodes is approximately 250mm to 300mm, which is sufficient for most general commercial POS systems. However, for applications requiring a more extensive depth of field, such as airport baggage conveyor systems, this depth may not be adequate. An attractive alternative is to use special optical collimating elements to achieve minimal beam divergence, thus achieving a more extensive depth of field.
(2) Optical Scanning System
The laser beam emitted by the source must pass through a scanning system to create scanning lines or patterns. Full-angle barcode laser scanners commonly use two approaches: rotating prism scanning and holographic scanning. Holographic scanning systems offer advantages such as a compact structure, high reliability, and low cost. Since IBM first applied this technology in the Model 3687 scanner, it has gained widespread use and continues to evolve. It's expected to capture a growing market share.
Rotating prism scanning technology has a long history and is relatively mature. It employs a rotating prism to scan the beam, using a set of folding plane reflectors to change the optical path for multi-directional scanning. Current scanners, like the MS-700, employ prisms with different wedge angles on different prism faces to create multiple scan lines in one scanning direction. This leads to a high-density scanning pattern composed of multiple scan lines in multiple directions. This method may also help reduce the laser radiation hazards.
The concept of full-angle scanning was initially introduced to improve the flow rate in supermarkets and designed the corresponding UPC barcode. For UPC codes, the "X" scanning pattern can already achieve full-angle scanning in two scanning directions. With advancements in scanning technology, the expansion of barcode application areas, and the urgent need for increased automation, this concept is now being extended to other barcode standards such as Code 39 and Interleaved 2 of 5. These barcode symbols are smaller in terms of height compared to width, and achieving full-angle scanning in multiple directions will require many more scan lines. To accomplish this, an additional moving element, such as a rotating folding plane mirror group, needs to be added to the system.
Handheld single-line scanners offer many solutions for beam scanning due to their lower scanning speed and smaller scanning angle. In addition to rotating prisms and oscillating mirrors, beam scanning can be achieved using various components in the moving optical system, such as moving semiconductor lasers and moving collimating lenses. The power elements that drive these movements can be DC motors, piezoelectric ceramics, and electromagnetic coils, among others. These power elements are known for their durability, long lifespans, and ease of use, and are expected to find applications in various industries.